Exemplary embodiments of the present invention relate to a method for operating a metal detection system that uses at least two operating frequencies and to a metal detection system that implements this method.
An industrial metal detection system may be used to detect and reject unwanted metal contamination. When properly installed and operated, it will help reduce metal contamination and improve food safety. Most modern metal detectors utilize a search head comprising a “balanced coil system.” Detectors of this design may be capable of detecting all metal contaminant types including ferrous, nonferrous, and stainless steels in a large variety of products such as fresh and frozen products.
A metal detection system that operates according to the “balanced coil” principle typically comprises three coils that are wound onto a non-metallic frame, each exactly parallel with the other. The transmitter coil located in the center is energized with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed with same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the first receiver coil is connected in series with the second receiver coil having an inverse winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity, cancel out one another in the event that the system, in the absence of metal contamination, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils, the voltage induced in each receiver coil is changed (by nano-volts). This change in balance results in a signal at the output of the receiver coils that may be processed, amplified, and subsequently be used to detect the presence of the metal contamination.
The signal processing channels split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known, then the corresponding signal vector may be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the contaminants, the product, and other disturbances have different influences on the magnetic field so that the resulting signals differ in phase.
The signals caused by various metals or products, as they pass through the coils of the metal detection system, may be split into two components, namely resistive and reactive components, according to the conductivity and magnetic permeability of the measured object. For example, the signal caused by ferrite is primarily reactive, while the signal from stainless steel is primarily resistive. Products that are conductive typically cause signals with a strong resistive component.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g., a frequency mixer or analog multiplier circuit, generates a voltage signal that represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants may be detected. However in the event that the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component.
In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
GB2423366A discloses an apparatus that is arranged to switch between at least two different operating frequencies such that any metal particle in a product will be subject to scanning at different frequencies. The frequency of operation is rapidly changed so that any metal particle passing through on a conveyor belt will be scanned at two or more different frequencies. In the event that for a first operating frequency the signal component caused by a metal particle is close to the phase of the signal component of the product and thus masked, then it is assumed that for a second frequency, the phase of the signal component caused by the metal particle will differ from the phase of the signal component of the product so that the signal components may be distinguished. By switching between many frequencies, it is expected that one frequency will provide a suitable sensitivity for any particular metal type, size, and orientation. U.S. Pat. No. 5,994,897A may operate in a similar manner.
Looking at these methods from a different angle, it may be stated that for one optimal frequency setting numerous other frequency settings have been applied, disclosing that this method requires considerable efforts. Various frequency settings need to be applied when measuring a single product. This means that for the frequency setting that provides the best result, only a small measurement period is available. Consequently, the result of the measurement will not be optimal. Furthermore, since the measurement is performed for all selected frequency settings, the major part of the data, which is processed with considerable efforts, will be disregarded. Hence, this method, which requires considerable efforts in the signal processing stages, is characterized by a relatively low efficiency.
Moreover, such metal detection systems that operate at different frequencies typically have a low sensitivity. Hence, although signals of metal contaminants may be obtained with a desirable phase, the detection of these signals may still fail due to the low sensitivity of such metal detection systems.
Also known in the art are metal detectors such as described in U.S. Pat. No. 6,724,191 B1, to Larsen, which discloses various circuits including an H-bridge switch network and a pulse width modulated switched capacitor resonator, for simultaneously resonating at several frequencies.
Similar to GB2423366A, GB2462212B refers to metal detectors that contain a drive circuit comprising four switches arranged as a full bridge circuit, wherein the coil system is connected across the output of the bridge. A programmable logic device controls the switches via a plurality of drive maps stored in the programmable logic device, with each drive map containing a switching sequence for a respective predetermined frequency of operation.
U.S. Pat. No. 5,859,533 to Gasnier describes an electromagnetic tomographic emitter for operating at variable frequencies to detect subsurface characteristics.
U.S. Pat. No. 5,304,927 discloses a method and apparatus for detection of metal in food products as packages of said food products are passed through the detector on a conveyor.
Exemplary embodiments of the present invention are therefore based on providing an improved method for operating a metal detection system that uses at least two operating frequencies as well as on providing a metal detection system adapted to operate according to this method.
Particularly, an exemplary embodiment of the present invention is based on providing a method that allows the detection of contaminants, particularly metal contaminants, with reduced efforts and a high efficiency.
Further, an exemplary embodiment of the present invention is based on providing a method that allows detecting small sized metal contaminants with higher sensitivity.
Still further, an exemplary embodiment of the present invention is based on providing a method that provides information about the capability of the metal detection system that may advantageously be used for the automatic configuration of the system.
An exemplary embodiment of the method serves for advantageously operating a metal detection system that comprises a balanced coil system including a transmitter coil that is connected to a transmitter unit, which provides transmitter signals with a selectable transmitter frequency, and a first and a second receiver coil that provide output signals to a receiver unit, which compensate one another in the event that the metal detection system is in balance and, in the event that product is present in the balanced coil system, provide an output signal that is forwarded to a signal processing unit, which suppresses at least the components of the product signal and delivers the signal components caused by metal contaminant contained in the product.
An example of the method comprises the steps of: determining the phase and magnitude of the related signals at least for a first metal contaminant for at least two transmitter frequencies and for at least two particle sizes of the first metal contaminant; determining the phase and magnitude of the related signal for a specific product for the at least two transmitter frequencies; comparing the information established for the at least first metal contaminant and the information established for the product; determining at least one preferable transmitter frequency with which the signal components of smallest sized particles of the at least first contaminant differ most in phase and amplitude from the phase and amplitude of the product signal; and selecting the preferable transmitter frequency for measuring the specific product.
An exemplary embodiment of the method may therefore allow obtaining optimal transmitter frequencies with which the smallest possible particles of one or more metal contaminant types may be detected. Accordingly, an example of a metal detection system may optimally be configured for any measurement, involving products of any consistency and any potential metal contaminant type.
In an exemplary embodiment, measuring a product at unsuitable transmitter frequencies and analyzing the related data may be avoided. One exemplary embodiment of the method may always apply the optimal frequencies so that measurements are performed with reduced efforts and high efficiency. For example, since measurements are not performed at unsuitable transmitter frequencies, the time available for measuring a product, i.e., for detecting metal contaminants in a product, is dedicated to the application of one or more optimal transmitter frequencies. As a result, more measurement data of high-quality is available for an individual metal contaminant. In an exemplary embodiment, this consequently leads to a significant improvement of the sensitivity of the metal detection system for all products and metal contaminant types measured. Optimal transmitter frequencies may therefore be determined for all metal contaminant types that may occur in a product and for all available products for all transmitter frequencies that may be selected.
In an exemplary embodiment, at least two curves of a first array at least for a first metal contaminant are established. Each curve is established for a separate transmitter frequency representing the phase and magnitude of the signal for a progressively increasing particle size of the first metal contaminant. Hence, a curve or response locus may be established at least for the first metal contaminant for at least two separate transmitter frequencies that are used as fixed parameters and with the particle size as a variable parameter. Each curve established for a specific transmitter frequency may be part of a first array that relates to the first metal contaminant. For each metal contaminant, a first array with at least two curves may be established.
The information established at least for the first metal contaminant and the information established for the product for at least a first and second transmitter frequency may then be compared in order to determine the preferred transmitter frequency, for which the signal components of smallest sized contaminant particles differ most in phase and amplitude from the phase and amplitude of the product signal.
In the event that information has been gathered for each transmitter frequency for more than one metal contaminant, then the complete information established for all metal contaminant types for a first and second transmitter frequency may be compared with the information of the product established for this first and second transmitter frequency.
For this purpose in an exemplary embodiment, for each transmitter frequency a second array may be built with curves of different metal contaminant types recorded with the same transmitter frequency. Then for a specific transmitter frequency a superposition of a second array and the product information may be arranged, which allows determining which parts of the curves lie within or outside of the area or range of the product signals. In an example, parts of the curves that lie outside the range of the product signals indicate particle sizes of the metal contaminant, which may not be masked or suppressed together with the product signals.
Hence, an exemplary embodiment of the method and the metal detection system may not only allow determining the optimal transmitter frequency of a metal contaminant but also allow determining the minimum particle sizes of the metal contaminant types that may be detected. This valuable information may be used for configuring an example of the metal detection system to operate most efficiently.
For instance, an operator may input the metal contaminant type that shall be detected. Based on this information provided by the operator, a computer system/program implemented in the metal detection system may often find a transmitter frequency that is suitable for the detection of two or more metal contaminant types. During the measurement process, an exemplary embodiment of the metal detection system may therefore be configured and operated with one of at least two transmitter frequencies that preferably meets all requirements set by the operator.
In the event that a single transmitter frequency does not satisfy the requirements of the operator, then the computer system/program may select two or more transmitter frequencies that are optimal for individual metal contaminant types and that are applied during the measurement of the product. The selected frequencies may then be applied according to a suitable method. The selected transmitter frequencies may be applied alternately or simultaneously, e.g., as a mixture of the selected transmitter frequencies, which may be filtered accordingly in the receiver stage.
Hence, in an exemplary embodiment, only optimal transmitter frequencies may be applied that allow measurement of metal contaminants for the maximum available time so that contaminants may be detected with the highest possible sensitivity.
In an exemplary embodiment, the operating system/program may be designed in such a way that the operator may input the minimum particle sizes for the metal contaminant types that shall be detected. This allows the operating system/program to select a transmitter frequency that is suitable for two or more contaminant types, for which the specified particle size may be detected.
In an exemplary embodiment, the required information for the metal contaminant types and the product may be gathered in various ways. For example, information may be pre-stored and downloaded. Alternatively, a calibration process may be performed for the product and the metal contaminant types, in which, e.g., a product and metal contaminants of at least one particle size may be measured.
Product information may be obtained when scanning a product, for which various signal components typically occur that have an individual phase and magnitude. For example, connecting the vectors of all signal components may lead to an envelope that is the boundary of an area of the product signals or the product signature that may be suppressed by a signal discriminator, e.g., a signal processor that is programmed to suppress the components of the product signal. In an exemplary embodiment, the area, in which signals of the product and contaminants are suppressed, may be closely adapted to the product signature but typically slightly larger, so that a safety margin may be provided. The product signature may change from transmitter frequency to transmitter frequency. For a general product, an algorithm, based on empirical data, may allow establishment of the product information preferably based on only one measurement performed at a single transmitter frequency in one exemplary embodiment.
In an exemplary embodiment during a setup period in the factory or before the start of a measurement process, data of metal contaminant types may be gathered by measuring metal particles with at least one particle size. Preferably, in one exemplary embodiment, only one or a few points of the curve may be measured, while the remaining part of the curve may be obtained by applying empirical data that is typical for that metal contaminant. In other exemplary embodiments, mathematical models or formulas may be used to establish the curves (e.g., by extrapolation) and/or sections between two measured points may be interpolated. In this way, an example of the calibration of an exemplary embodiment of a metal detection system may require only a few or other limited number of measurements that provide at least the starting points of the curves or first and/or second arrays.
In one exemplary embodiment, the gathered information is preferably stored in a memory of the control unit or a computer system that is attached to or integrated into the metal detection system. The information stored may then selectively be downloaded and used for the future calibration and configuration of the metal detection system.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.